1. Field of the Invention
The invention relates to a system for minimizing depolarization due to thermally induced stress birefringence in a solid state gain medium used in laser amplifiers and resonators.
2. Description of the Prior Art
Excitation of the gain medium, such as a Nd:YAG rod, used in high power laser amplifiers generates significant heat that thermally distorts the medium and induces stress birefringence.
This creates depolarization losses in polarization dependent cavities. By implementing a birefringence compensation scheme, an increased output power and a higher polarization contrast ratio can be obtained over the uncompensated cavity. A well-known method of birefringence compensation is to use two identical cylindrical rods spaced by a 90xc2x0-quartz rotator. In this scheme, the light""s polarization is rotated 90xc2x0 between the two rods so that the beam""s radial (r) and tangential (xcfx86) polarizations are exchanged and thus each experience both indices of refraction, nr and nxcfx86. However, because the polarizations focus at different rates after the first rod, the r and xcfx86 mode volumes become non-equivalent in the two rods. Consequently, the birefringence compensation becomes incomplete and less effective with increased pump powers. Lu et al. identified this problem and offered a solution by including an imaging optic into the above compensation scheme. [Q. Lu, N. Kugler, H. Wever, S. Dong, N. Muller, and U. Wittrock, xe2x80x9cA novel approach for compensation of birefringence in cylindrical Nd:YAG rods,xe2x80x9d Opt. and Quantum Elect. 28, 57-69 (1996).] With this imaging optic, theoretically perfect birefringence compensation is possible at all pump powers.
Another compensation scheme is detailed in U. S. Pat. No. 5,504,763. A phase conjugate mirror is used to reverse wavefront distortions. At high laser power significant birefringence is created that induces strong depolarization effects in the beam. Since the phase conjugate mirror is dependent on the polarization purity of the incoming beam, a relay imaging means, a faraday rotator, and a flat reflector are used to compensate for the birefringence caused depolarization. While this system discloses the concept of using a relay imaging means in the birefringence scheme, it remains relatively complex without reference to the use of a single concave mirror with a precisely defined position. In fact, the ""763 patent makes reference to the simplest embodiment as two identical lenses with a mirror. Additionally, the present invention further distinguishes itself from this patent with the disclosure that the r and xcfx86 polarized modes are of equal size on the output side of the gain medium. The present invention further discloses that this fact can be used for spatial mode control and the development of ring laser and amplifier designs. [C. A. Denman and S. I. Libby, xe2x80x9cBirefringence compensation using a single Nd:YAG rod,xe2x80x9d OSA TOPS Vol. 26, 608-612 (1999) Advanced Solid-State Lasers, Martin M. Fejer, Hagop Injeyan, and Ursula Keller (eds.)]
Accordingly, it is an object of the present invention to introduce a simplified single gain medium (pump-head) implementation of a birefringence compensation scheme that minimizes the thermally induced depolarization loss. It is another object of the present invention to disclose attributes of the system and to provide an approach for optimal use of the gain medium volume and mode size for spatial mode control. It is a further object of the present invention to provide an approach for using the subject birefringence implementation in a ring laser resonator and amplifier configuration.
The present invention is a single pump-head birefringence compensation scheme using a 45xc2x0 Faraday rotator located between the pump head (Nd:YAG rod) and a concave mirror. The concave mirror serves as a focusing element that ensures that each ray of the laser beam is reflected directly back on itself and passes through the pump head rod in exactly the same position as its incident path. Therefore, the properties of the reflected r and xcfx86 polarized modes will be the same as the original modes. Upon a double pass of the 45xc2x0 Faraday rotator, the polarization of the beam is rotated by 90 degrees and the beam""s radial (r) and tangential (xcfx86) polarizations are exchanged. Consequently, equivalent mode volumes are reciprocated and each laser beam ray experiences both indices of refraction with equal focus upon completion of a double pass of the rod. In this manner, the r and xcfx86 polarized modes become identical (their mode spot size and radius of curvature as a function of position become identical not their polarization state) after a double pass of the rod. Because of this fact, resonator or amplifier optics on this side of the rod can be changed or adjusted to alter the modes volume within the rod so as to optimize the laser""s spatial-mode performance without affecting or changing any of the rotation is used before reentering the rod, the lasing mode within the rod does not form a standing wave, thus alleviating the problem of spatial hole burning. Hence, a polarized birefringence compensated single-frequency lasing mode oscillates as though it is in a standard ring oscillator configuration. A Y-cavity implementation and an injection-locked laser ring configuration are presented.